As a use of renewable energy rapidly increases, a need for an energy storage device using a battery rapidly increases. As the battery, a lead battery, a nickel/hydrogen battery, a vanadium battery and a lithium battery may be used. However, the lead battery and the nickel/hydrogen battery have significantly low energy density which is a problem in that a large space is required to store energy having the same capacity. In addition, the vanadium battery has a problem in that environmental pollution is caused by using a solution containing heavy metals, and amounts of anode and cathode materials that are moved through a membrane separating the anode and the cathode are small, which causes deterioration of performance, such that mass-product ion of the vanadium battery has not been achieved yet. The lithium battery having significantly excellent energy density and output characteristic is technically advantageous. However, it is difficult to be economically used as a secondary battery for large scale power storage due to scarcity of a resource of a lithium material.
To solve this problem, there are a number of attempts to use sodium, which is an abundant resource on earth, as the material of the secondary battery. Among them, a sodium-sulfur battery that uses a solid electrolyte having selective conductivity with respect to sodium ions, and has a form in which an anode is supported with sodium, and a cathode is supported with sulfur, or a sodium-transition metal halide battery that uses a solid electrolyte having selective conductivity with respect to sodium ions, and has a form in which an anode is supported with sodium, and a nickel (Ni) cathode is supported with a molten sodium cathode solution containing sodium salt and NaAlCl4 has been used as a device for large scale energy storage, as described in U.S. Patent Application Publication No. 20030054255.
However, the solid electrolyte used in the sodium secondary battery has a problem in that resistance is increased due to low wettability with respect to molten sodium at a temperature of 250° C. or less, to deteriorate battery efficiency.
In particular, the low wettability of the solid electrolyte that separates the cathode and the anode requires a larger amount of active materials, which causes an increase of battery cost and deterioration of battery performance.